1. Field of the Invention
The present invention generally relates to a method for driving a fluorescent lamp and a circuit for performing such a method, and more particularly to a method for driving an inverter circuit for the fluorescent lamp to reduce an instantaneous power source loading and amount of electromagnetic interference (EMI) generated by a transformer.
2. Description of Related Art
FIG. 1 is a conventional inverter circuit including a full bridge converter, which comprises a DC (direct current) voltage source 110, a bridge DC/AC (alternative current) converter 120, a transformer 130, a CCFL 140, an LCD 145, a voltage sensor 160, a current feedback 150 and a feedback control unit 170. The bridge DC/AC converter 120 comprises a switch A, a switch B, a switch C and a switch D, each of which comprises a metal-oxide-semiconductor field effect transistor (MOS FET) and a diode connected in parallel. More, the feedback control unit 170 comprises an error amplifier & control circuit 171, a drive circuit 173 and a pulse-width modulator 175. Furthermore, the CCFL 140 is disposed in the liquid crystal display 145.
One group comprised of the switch A and switch D and the other group comprised of the switch B and the switch C are alternatively turned on in accordance with a pulse signal provided by the drive circuit 173, whereby a DC square wave voltage outputted from the DC voltage source 110 is converted to an AC square wave with a high frequency. There occurs a voltage difference between nodes P1 and P2, which is an output of the bridge DC/AC converter 120. The DC square wave with a high frequency is then converted to an AC quasi-sine wave signal with a high frequency and a high voltage for driving the CCFL using the transformer 130 and capacitors C1 and C2.
Subsequently, the current feedback 150 senses a current signal passing through the CCFL 140, the voltage sensor 160 senses a voltage signal inputting to the CCFL 140 from a secondary winding of the transformer 130 and eventually the feedback control unit 170 proceeds with a negative feedback in accordance with the current signal and the voltage signal. Since a brightness of the CCFL 140 depends on a magnitude of a current passing through it, the error amplifier & control circuit 171 can compare the current with a predetermined value and output a range of control signals to a pulse-width modulator (PWM) 175 in accordance with a magnitude of a deviation of the current. An adjusted pulse-width AC square-wave signal can be obtained at a primary side of the transformer 130 by using the pulse-width modulator 175 and the drive circuit 173 to control a pulse-width of the output signal of the bridge DC/AC converter 120. The adjusted pulse-width AC square wave signal is then transformed to an AC quasi-sine waveform signal by the transformer 130 and the second capacitor C2, which in turn is inputted to the CCFL 140, thereby achieving a purpose of stabilizing and adjusting the brightness of the CCFL 140.
The detail operation of how to obtain the adjusted pulse-width of the AC square-wave output signal of the bridge DC/AC converter 120 is described as follows. After the AC quasi-sine waveform signal passes through the CCFL 140, the current feedback 150 senses a current signal outputted from the CCFL 140, and the voltage sensor 160 senses the AC quasi-sine waveform signal as well. Then, the error amplifier and control circuit 171 outputs a feedback control signal to the drive circuit 173 in accordance with the current signal outputted from the CCFL 140 and the AC quasi-sine waveform signal. Subsequently, the drive circuit 173 outputs an adjusted pulse-width driven signal to the bridge DC/AC converter 120, which in turn outputs an AC adjusted pulse-width square wave to the primary side of the transformer 130, thereby forming a negative feedback loop for driving the CCFL 140. Subsequently, the AC adjusted pulse-width square wave is converted to an AC quasi-sine wave for driving the CCFL 140, thereby achieving a purpose of stabilizing and adjusting the brightness of the CCFL 140.
FIG. 2 shows voltage timing charts present at several components in the circuit shown in FIG. 1, from which it can realized that how an AC square wave with an adjusted pulse-width is obtained from the bridge DC/AC converter 120. In FIG. 2, WAV_A, WAV_B, WAV_C and WAV_D show turn-on timing charts of the switch A 121, the switch B 123, the switch C 125 and the switch D 127, respectively, wherein in WAV_B, the term of “B_ON” stands for an on-state of the switch B; likewise, the similar terms apply to WAV_B, WAV_C, WAV_D. More, WAV_E shows a dead-time timing chart generated from the drive circuit 173, by which the switch A 121 is turned off and the switch B 123 is turned on after a while i.e. the switch A and the switch B are not turned on at the same time due to a transition state period from a low level to a high level or from a high level to a low level. As a result, pulses D3 and D4 can prevent the switches A and B from being turn on simultaneously. WAV_F chart shows “switching timing” of the bridge DC/AC converter 120, wherein the term of “B and C_ON” stands for the switched B and C being turned on simultaneously and the term of “A and D_ON” stands for the switched A and D being turned on simultaneously. As an operation of the full bridge DC/AC converter 120, it can convert a DC voltage output from the DC voltage source 110 to an AC square wave by alternatively turning on one group switches consisted of switches A and D and the other group switches consisted of switches B and C in accordance with pulses provided by the drive circuit 173.
In addition, “Primary Driving Voltage” shows an AC square wave with a positive voltage VCC1 and a negative voltage −VCC1 outputted from the bridge DC/AC converter 120 to the primary winding of the transformer 130. Finally, “Secondary Voltage” shows an AC quasi-sine waveform signal present at a joint node between the second capacitor C2 and the CCFL 140.
Obviously, from “Primary Driving Voltage” in the FIG. 2, an instantaneous loading of the DC voltage source 110 is too high because it is used to generate a single-level square wave with a high voltage VCC1. If the instantaneous DC voltage source 110 is used to generate a two-level or multi-level square wave, its loading can be alleviated due to a smaller voltage variation. Besides, electromagnetic radiating wave generated by the transformer 130 can interfere other components in a mother board, which results in an electromagnetic interference (EMI) phenomenon. In addition, EMI also affects a read/write malfunction of a CPU. Most importantly, the bridge DC/AC converter 120 is particularly susceptible to EMI. Once the bridge DC/AC converter 120 is interfered by EMI, it cannot function normally so that a stabilized operating current for driving the CCFL cannot be obtained, which causes the CCFL 140 to have an unstable brightness. Also, the CPU interfered by EMI causes a computer, such as a notebook computer and a palm computer, to have a malfunction.
Therefore, it is needed to provide a method for alleviating EMI generated by the transformer in the inverter circuit for driving the CCFL in a field of manufacturing a liquid crystal display. Furthermore, by reducing amount of EMI generated by the transformer, a purpose of maintaining a stable brightness of the CCFL can be achieved.